System requirements/How-TO Assumes:

http://dri.sf.net (never went to the website, but used their CVS repository for the DRM necessity)

MythTV-0.19-fixes

Desire for Diskless frontend

Instructions

Install Gentoo (beyond the scope of this document, but well documented at http://gentoo.org). After doing a minimal install, go back, unmask the GCC-4.1 ebuilds, re-emerge what you have already, and then start from scratch one more time (make sure after you install GCC-4.1 you use gcc-config to switch your profile to that compiler). Now we should have a minimal system completely built with GCC-4.1. Next, emerge the kernel and include the following options:
Root filesystem on NFS, AGPGart (M), via-agp (M) leave DRM alone, we'll deal with that outside of portage later. You'll need networking stuff, specifically your Ethernet card needs to be built into the kernel as well as Kernel Level IP Configuration options (DHCP, BOOTP, etc), not modularized. Don't forget you need a Cron and system logger, etc, but don't blindly rc-update add everything, yet..

Next we'll move onto getting Xorg up. Following This Guide,
you can get the Modular X up and running with little hassle. Simple emerging of a few obvious files (you need a keyboard working right?)
Then you need a driver for your card. Here's where things go a little rough:

If you've got a newer UnichromePro chip, such as the P4M800CE, you'll need to grab the OpenChrome drivers along with DRM and hand edit a file or 2, so according to this page, We will:

This should pull up both drm and via, and in your dmesg give you some sort of message about DRM initializing via (this message varies widely it would seem).

Now, cross your fingers and:

startx

Edit: If you get an error (EE) No Devices Detected when you start Xorg you need to check the location on the driver. Running make install installs in /usr/local but X looks in /usr/. Depending on your steps so far, a via driver may already have been built (which means you won't be told the driver hasn't been found). Simply copy the compiled unichrome driver (via_drv.so) over the top of the one that X is trying to use.

Hopefully you are in and:

glxinfo | grep direct

Returns a few lines, and one of them says:

direct rendering: Yes

Now, emerge (or re-emerge if you were impatient) mythtv, et al:

emerge -av mythtv mplayer xine-lib

It's necessary (for everyone with Via chipsets?) to patch mplayer to enable XvMC on via hardware. I also had to edit the mythtv ebuild to enable xvmc-pro as an option, without providing a hacked ebuild, here's the idea:
Open up /usr/portage/media-tv/mythtv/mythtv-0.19.ebuild and add:
xvmc-pro as a USE option;
Next find then section where --enable-xvmc is located, and add:

NOW emerge mythtv, hopefully not again, because you read things to the end before you start right? ;)

Should be about it, I'm sure I'm forgetting a very important step, but that's about all I have down. In the end, if problems arise, check the openchrome mailing list or the mythtv mailing list, both are cornucopias of information.